e.g., open firebug making the admin really small, vertically; then click the logout button.
This is a known issue on the Bootstrap issue tracker.
It is also a size problem, the dialog can be higher that the screen.
It should have a scrollbar in that case.
mod_base: when a dialog is heigher than 80% of the window, reposition…
mod_base: better positioning of popup dialog on narrow screens.